The Diagram has parameters midX and midY to indicate where to display pieces in move diagrams (a1 = (0,0)). By default they are set to the middle (rounded down for rank, and up for file).
Problem is that these defaults are only set when you use the files= and ranks= parameters (and midX / midY should occur after these to have effect). If you leave out these parameters, relying on their default value of 8, the piece would be displayed at b2.
The Diagram has parameters midX and midY to indicate where to display pieces in move diagrams (a1 = (0,0)). By default they are set to the middle (rounded down for rank, and up for file).
Problem is that these defaults are only set when you use the files= and ranks= parameters (and midX / midY should occur after these to have effect). If you leave out these parameters, relying on their default value of 8, the piece would be displayed at b2.